Polyethylene terephthalate is a low cost polymer but until recently it had not been accepted as a good molding resin for a number of reasons. For example, certain alkali metal salts of organic carboxy containing compositions (alkali metal organic carboxylates) are commonly added to promote crystallization by enhancing nucleation of the polyethylene terephthalate. In addition, polyethylene terephthalate is commonly flame retarded by adding common flame retardants such as halogenated organic compounds and an antimony oxide synergist. But, when flame retardant systems containing an antimony oxide synergist are added to a blend of polyethylene terephthalate and alkali metal organic carboxylates, the polymer is degraded, resulting in a deterioration of mechanical properties and a decrease in melt viscosity. This deterioration when antimony oxide is present is particularly unfortunate since antimony oxide is a particularly good synergist for halogenated aromatic organic flame retardants. It is readily available, is low cost, and is highly effective. In contrast, many other inorganic synergists are simply not effective in polyethylene terephthalate flame retardant systems or degrade the resin when used with polyethylene terephthalate, some of these being zinc oxide, zinc sulfide, zinc borate, stannous oxide, molybdenum oxide and molybdates.
A flame retardant system using antimony oxide which, when added to the blend of polyethylene terephthalate and alkali metal organic carboxylates, would not substantially affect the thermal stability of the polyethylene terephthalate is desirable, and is provided by this invention.